Automotive radar products are typically made by assembling a number of discrete components on one or more printed circuit boards. This results in automotive radar products that are typically undesirably bulky. Additionally, current narrowband frequency modulated continuous wave (FMCW) automotive radar products transmit a signal with a frequency ramp in discrete frequency steps. The discrete frequency steps are created using a digital-to-analog converter (DAC) integrated circuit to tune a free-running voltage controlled oscillator (VCO). The DAC is generally located on a Digital Signal Processor (DSP) module and the tuning voltage is communicated from the DSP module to a separate printed circuit board containing the VCO. Unfortunately, traversing a board-to-board connection makes the tuning voltage more susceptible to noise.
The VCO typically requires frequency calibration over a range of temperatures because the oscillator frequency varies with the temperature. Generally, a free running VCO is calibrated by comparing its frequency to the frequency of a second VCO, which is referred to herein as a calibration VCO. The calibration VCO, in turn, is phase locked to a crystal oscillator to generate a reference frequency. When the free-running oscillator is swept over a frequency range which encompasses the reference frequency, a mixer translates the reference frequency to direct current (DC), causing an impulse in the baseband filter. The impulse can be observed using an analog-to-digital converter to determine the free-running oscillator's tuning voltage that corresponds to the reference frequency.
With reference to FIG. 1, a prior art FMCW radar system 100 comprises a transmit portion 110 and a receive portion(s) 120 of a radio frequency (RF) module 199. Furthermore, transmit portion 110 and receive portion(s) 120 are in communication with a digital signal processor (DSP) module 130. The transmit and receive portions typically comprise discrete components. However, assembling the discrete components of RF module 199 results in an overall size increase in comparison to highly integrated circuit architecture.
Furthermore, the typical design of radar system 100 includes a free running VCO 101 and a phase locked loop (PLL) architecture 150. A typical PLL architecture 150 comprises a calibrating VCO 152, a reference crystal oscillator (XSTL) 151, a PLL 153, and a loop filter 154. Calibrating VCO 152 operates at a known frequency due to being in a phase lock loop with XSTL 151.
In addition, PLL architecture 150 is generally isolated from free running VCO 101 on RF module 199 because the output of calibrating VCO 152 is multiplied and operates at the same, or approximately the same, frequency as free running VCO 101. As is well known, having signals with similar frequencies in close proximity may result in signal interference and noise.
With continued reference to prior art FIG. 1, one manner of tuning free running VCO 101 involves VCO tuning circuit on DSP module 130 comprising a pulse width modulator (PWM) 131 and a DAC 133. The VCO tuning circuit is adjusted based on a comparison of the output of VCO 101 and the multiplied output of calibration VCO 152.
However, using a calibration VCO to generate a reference frequency adds cost to the system and occupies valuable space. Thus, a need exists for a device with improved isolation of a tuning voltage for a VCO and an improved method of frequency calibrating a VCO. This invention addresses these needs and others.